1. Field of the Disclosure
The present disclosure relates to a fluid filtering device and, more particularly, to a pleated filter element and a method of forming a pleated filter element. Even more particularly, the present disclosure relates to a pleated filter element having a modified W-pleat construction, and a method of forming such a filter element.
2. Background of the Related Art
Filtration is the process of separating particles, or contaminants from a fluid (liquid or gas), and can be accomplished by passing the fluid through a porous filter medium that stops or captures the particles while permitting the fluid to pass there through. Such fluid filtering is used extensively in the manufacture of polymer products, medicinal products, mineral and metallurgical processing, petroleum refining, water purification, emissions control, and in beverage and food preparation.
Most conventional filter media may be categorized into two broad categories: a surface-type filter medium and a depth-type filter medium. As its name implies, the surface-type filter medium stops fluid contaminants on its surface. Examples of surface-type filter media are calendered melt-blown material, cellulose and/or paper, membranes, woven screen, porous metal, and porous non-woven material. In contrast, a depth-type filter medium captures contaminants within the medium, i.e. between an upstream surface and a downstream surface of the medium. An example of a depth-type filter medium is a resin bonded filter.
In general, as a fluid is forced through the filter medium and filtered, over time the pressure drop across the filter medium will gradually increase. Such increase is due to the collection of particles, or contaminants on the filter medium, i.e., the filter medium gradually becomes loaded with the contaminants trapped or stopped thereby. An increasing pressure drop across the filter medium, however, translates into an increasing load on the means (such as a blower or pump) employed to force the fluid through the filter medium. In addition, since the life of a filter is generally defined by a maximum allowable pressure drop, a slower increase in pressure drop translates into a longer filter life.
When a surface-type filter medium is used, one method of minimizing the pressure drop across the filter medium is to maximize the available surface area of the filter medium. In order to increase the surface area in a surface-type filter, pleated filter media have been developed. Pleated surface-type filters typically include relatively thin cellulosic or synthetic filter media that is folded in an accordion-like fashion to produce a plurality of pleats. Each pleat is typically made up of a pair of rectangular panels, with fold lines separating the panels.
In a cylindrical pleated filter element, the short sides of the rectangular panels of the pleats usually extend radially outwardly with respect to the axis of the filter element, and thus provide the radial height of the pleats, while the long sides of the rectangular panels of the pleats extend axially between ends of the filter element. The maximum number of full pleats (i.e., pleats that extend between the inner and outer diameters of the filter element) is determined by an inner circumference of the filter element divided by the thickness of the pleats.
Because of the radial geometry of the pleats in a cylindrical pleated filter element, however, there is a significant degree of spacing between outer tips of the pleats. In order to minimize the spacing between outer tips of the pleats, filter elements having larger inner diameters and, thus, shorter pleat heights have been used. Furthermore, spiral pleat filters and "W-pleat" filters have been developed in order to minimize pleat spacing and provide even more filtering surface area.
A W-pleat filter element is comparable to a standard pleated filter element in that it includes a plurality of longitudinal pleats disposed in a cylindrical configuration. The W-pleat filter element, however, also includes relatively short pleats extending radially inward from the outer periphery of the filter between the pleats of standard height. The short pleats are the same height and arise at a uniform frequency about the circumference of the filter, i.e., there is one short pleat between every two full-length pleats. Examples of W-pleat filters can be found in U.S. Pat. Nos. 2,627,350 (1953) to Wicks; U.S. Pat. No. 3,002,861 (1962) to Harms; 3,799,354 (1974) to Buckman et al.; and German Patent No. 3,935,503 (1991) to Nick et al. Most W-pleat filters are made using cam-actuated pleating machines that only provide repetitive and uniform pleat patterns, resulting in short pleats of the same height and arising at a uniform frequency.
One problem associated with the W-pleat construction, however, is a less than optimum pleat density and the migration of the shortened pleats towards the axis of the filter. Such migration is undesired because it can cause binding, blockages, increased pressure drops across the filter, reduced filter lives, and damage the filter media.
A spiral pleat filter element is comparable to a standard pleated filter in that it includes a plurality of longitudinal pleats disposed in a cylindrical configuration. In a spiral pleat filter, however, the ends of the pleats are rolled over to minimize the spacing between adjacent pleat surfaces near an outer diameter of the filter element, such that more filter surface area can be provided in a filter of equal diameter. Examples of spiral pleated filters can be found in U.S. Pat. Nos. 2,395,449, 2,401,222 and 2,420,414 (1946) to Briggs; 2,801,001 (1957) to Bowers; and 5,543,047 (1996) and 5,690,765 (1997) to Stoyell et al.
While both the spiral pleat and the W-pleat designs provide surface-type filters with increased filter surface area, the spiral pleat designs do not have the pleat migration problems associated with the W-pleat designs. As compared with a W-pleat filter, however, the rolled-over pleats of a spiral pleated filter provide fewer and more difficult to access radial flow paths near the outer diameter of the filter, leading to a greater pressure drop across the filter. In addition, the rolled-over pleats of a spiral pleated filter provide longer flow paths and, therefore, a greater chance of the flow paths becoming blocked in high load or large particle contaminant applications.
Furthermore, spiral pleated filters are more difficult to axially insert into a cylindrical cage of a cartridge assembly incorporating the filter element, since the rolled-over pleats have a tendency to straighten out prior to being inserted into the cage. Inserting a spiral pleated filter element into a cage creates drag, which can cause damage to the filter media and can, as a practical matter, limit the axial length of a filter cartridge incorporating a spiral pleated filter element.
What is still needed, accordingly, is a filter element that provides filter surface area gains comparable to a spiral pleated filter, yet has increased radial flow paths at the outer diameter of the filter, and is conducive to being inserted into an elongated cylindrical cage. There is also a need for a pleated filter design that prevents pleat migration.